1. Field of the Invention
The present invention relates to a line thermal printer, and in particular relates to a line thermal printer which is suitable, for example, to be used with a terminal etc. and which is to be connected to a compact type portable equipment driven by batteries, for example.
2. Description of the Related Art
In recent years, a number of so called thermal printers have been developed, which print by electrically heating the print head corresponding to the letters and graphics on to a thermosensitive paper which develops colors if heat is sensed.
These thermal printers, especially a thermal printer driven by batteries are roughly classified into 2 groups,
(1) a serial thermal printer of a low voltage system (about 5 V), and
(2) a line thermal printer of a high voltage system (about 12 V) and generally, a thermal printer working with a low voltage is slow in print speed while a thermal printer high in print speed requires a high voltage.
In other words, compact portable equipment, for example, a terminal and the like can not print at a high speed because the equipment is driven by a battery in the order of 4.5 to 6.5 V or with a low voltage in a 5 V single power source.
Therefore, a line thermal printer that can be operated with a low voltage, for example, around 5 V is required.
On the other hand, there exist a variety of thermosensitive papers being used with this thermal printer, such as red/black paper the color of which changes, for example of example, different temperatures are sensed, different print energies, are given and a 2-ply paper is sensed. A 2-ply paper does not develop colors unless a greater print energy is provided than the usual print energy, which makes a 1-ply thermosensitive paper.
Consequently, in order to obtain favorable print quality for a variety of thermosensitive papers, a line thermal printer is necessary that can change the print energy per unit area in accordance with the thermosensitive papers to be used.
In the past, a line thermal printer as shown in FIG. 1, for example, is available as this type of thermal printer.
The print head 1 of this line thermal printer is structured of 4 print blocks, 3a to 3d, whose heating element 2 of 320 dots per dot line has been divided at every 80 dots.
In the aforesaid structure, if the printing is made on a given thermosensitive paper, the heating element 2 in the print block 3a is electrified for a stipulated time, for example, with a high voltage of around 12 V, first based on the stipulated data, the printing is made at the thermosensitive paper position (shaded area (1)) corresponding to the print block 3a as shown in FIG. 2, and then the paper is fed out very slightly.
Thereafter, the heating elements 2 in the print blocks 3b, 3c and 3d are sequentially electrified in a similar way, the printing is made on the thermosensitive paper positions (shaded areas (2), (3) and (4)) corresponding to the print blocks 3b, 3c and 3d, and thus the printing of 1 dot line is ended.
However, this type of conventional line thermal printer has been structured such that the printer is electrified for the stipulated time with a high voltage of around 12 V, so if the printing has been made at every line with a low voltage, for example, from 4.8 V to 5.0 V, there is the problematic point that a favorable print quality or print density can not be obtained and the printing can not be made at a high speed as a result because the print energy is small. This is because, compact portable a terminal and like, for example, need to be driven by a low voltage, or by a single power source of around 4.8 V. Sufficient print energy can not be produced with this type of low voltage.
In addition, there is the problematic point that the printing can not be made at a high speed with the drive at a low voltage.
In other words, to obtain the necessary to color the thermosensitive paper with a low voltage, the resistance value needs to be made lower and the current value needs to be made larger, but in general, the energy needed for printing is expressed by the following formula: ##EQU1## where .epsilon.=Energy per unit area.
v: Impressed voltage. PA0 Vs: Saturation voltage. PA0 R: Heating element resistance value. PA0 t: Pulse width. PA0 S: Heating element area,
and for enlarging the print energy,
(1) the saturation voltage "Vs" must be made smaller,
(2) the heating element resistance value "R" must be made smaller,
(3) the heating element area "S" must be made smaller, or
(4) the pulse width "t" must be made larger.
However, there is a limit to decrease the heating element resistance value "R" because of the material of print head. If the heating element resistance value "R" should become smaller, the current "i" flowing to the heating element 2 becomes greater, so the saturation voltage "VS" is enlarged. The saturation voltage "Vs" becomes an ineffective energy and has no effect on the print coloring.
This can be explained as EQU Current "i"=(V-Vs)/R,
but this is the current flowing to a single heating element, and the total current flowing to the actual print head 1 will become as follows: ##EQU2##
Namely, the saturation voltage Vs depends on the current "i" and becomes approximately as follows: EQU Vs=ai+b.
(For information, the voltage actually becomes non-linear from the saturation characteristics of the switching transistor).
Therefore, if the current "i" becomes greater, the saturation voltage Vs is increased, and because the formula V-Vs becomes smaller, the print energy ".epsilon." becomes smaller.
In addition, if a battery is used, it generates the new problematic point that the presence of internal impedance from using the battery becomes a printing problem as the drop in voltage because the impressed voltage "V" itself depends on the current "i" and therefore fluctuates with the loss inside the head due to the common conductor inside the head. For this reason, the optimum range of heating element resistance "R" is determined by itself, and consequently the pulse width "t" must essentially be set to relatively narrow value.
Enlarging the pulse width "t" results generally in enlarging the print cycle, so the print speed becomes slow. The pulse width "t" can not be set to a high value because realize a high speed printing is desirable.
Decreasing the pulse width generally results in decreasing the print cycle, so that the print speed can be accelerated. However, decreasing the pulse width involves the problematic point that the print energy ".epsilon." essentially becomes smaller.
Therefore, an object of this invention is to present a thermal printer to print at high speed with the drive of low voltage of around 4.8 to 5 V while preventing the printing troubles attributable to the fall in voltage coming from the internal impedance and to the loss in head due to the common conductor inside the head. and moreover,
it is another object to present a line thermal printer capable of obtaining the favorable print quality against a variety of thermosensitive papers while preventing the drop in print speed even in the case of low voltage, for example, of around 4.8 to 5.0 V.